(Corrects headline to tropical storm from depression)

MIAMI, Sept 7 (Reuters) - Tropical Storm Gabrielle formed on Friday off the U.S. mid-Atlantic Coast with top sustained winds of 45 mph (72 kph), but was not seen strengthening in the next 24 hours, the U.S. National Hurricane Center said.

Gabrielle, the seventh named storm of the Atlantic hurricane season, was about 385 miles (625 km) southeast of Cape Lookout, North Carolina, the Miami-based hurricane center said in its first advisory on the tropical storm at 11 p.m. (0300 GMT on Saturday).

"Little change in strength is forecast during the next 24 hours," the hurricane center said. Top winds must reach at least 74 mph (119 kph) for a tropical storm to become a hurricane.

A tropical storm watch was issued for portions of the North Carolina and South Carolina coasts. Gabrielle was moving west-northwest at near 10 mph (16 kph).

Computer models used to forecast future storm tracks indicated the weather system would most likely head northwestward toward the coast of North Carolina before looping around to the northeast and cooler waters.

It was very unlikely, however, the system could reach the top-rank strength of Hurricanes Dean and Felix, which slammed into Mexico's Yucatan Peninsula in August and Central America this week respectively as Category 5 hurricanes on the five-step Saffir-Simpson scale of hurricane intensity.

Sea surface temperatures and atmospheric conditions off the U.S. Southeast were nowhere near as favorable for tropical cyclones as in the western Caribbean, where Dean and Felix grew into monster storms, the hurricane center said earlier.

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